Method of making a blend of fogged,direct positive silver halide emulsions of different grain sizes

ABSTRACT

A METHOD OF MAKING A PHOTOGRAPHIC ELEMENT HAVING AT LEAST ONE LAYER CONTAINING A BLENDED GRAIN SILVER HALIDE EMULSION, WHEREIN THE AVERAGE GRAIN SIZE OF AT LEAST TWO COMPONENT EMULSIONS OF THE BLEND DIFFERS BY AT LEAST 53%, WHICH COMPRISES: PREPARING THE RESPECTIVE COMPONENT SILVER HALIDE EMULSIONS, BLENDING THE SILVER HALIDE COMPONENT EMULSIONS, COATING SAID EMULSION BLEND ON A SUPPORT AND REDUCING THE LIQUID CONTENT OF THE COATED BLEND; THE IMPROVEMENT WHICH COMPRISES DEFERRING THE BLENDING OF THE COMPONENT EMULSIONS OF DIFFERENT GRAIN SIZE UNTIL JUST PRIOR TO COATING AND THEREUPON ADMIXING THE COMPONENT BLENDS AND PROMPTLY THEREAFTER COATING THE FRESHLY FORMED ADMIXTURE ON THE SUPPORT WHEREBY THE SUPPORT IS COATED WITH THE MINIMUM POSSIBLE TIME DELAY BETWEEN THE TIME OF ADMIXING AND THE TIME OF COATING.

United States Patent 3,600,180 METHOD OF MAKING A BLEND 0F FOGGED, DI-

RECT POSITIVE SILVER HALIDE EMULSIONS OF DIFFERENT GRAIN SIZES MalcolmL. Judd and Fred W. Spangler, Rochester, N.Y., assignors to EastmanKodak Company, Rochester, NY. No Drawing. Filed Nov. 4, 1968, Ser. No.773,327 Int. Cl. G03c 1/02, 1 U.S. Cl. 96-94 6 Claims ABSTRACT OF THEDISCLOSURE A method of making a photographic element having at least onelayer containing a blended grain silver halide emulsion, wherein theaverage grain size of at least two component emulsions of the blenddiffers by at least which comprises: preparing the respective componentsilver halide emulsions, blending thesilver halide component emulsions,coating said emulsion blend on a support and reducing the liquid contentof the coated blend; the improvement which comprises deferring theblending of the component emulsions of different grain size until justprior to coating and thereupon admixing the component blends andpromptly thereafter coating the freshly formed admixture on the supportwhereby the support is coated with the minimum possible time delaybetween the time of admixing and the time of coating.

This invention relates to new processes for coating silver saltphotographic emulsions. In one embodiment, this invention relates to newmethods for coating admixtures of silver salt emulsions wherein saidadmixture comprises aliquot amounts of monodispersed grains of differentgrain sizes. In another embodiment, this invention relates to novelmeans for obtaining layers of silver halide emulsions having a finegrain emulsion in admixture with a coarse grain emulsion.

It is known in the art that extended latitude in photographic emulsionscan be obtained by mixing silver halides of various grain sizes, such asdisclosed inBritish Pat. 732,691. It is also known in the art thatOstwald ripening occurs when a fine grain silver halide emulsion ismixed with a coarse grain silver halide emulsion, the larger grainsgrowing at the expense of the smaller grains. Therefore, it has beenimpractical in many instances to use mixed grain size emulsions toobtain extended exposure latitude, especially Where long holding timeswere necessary in the coating operation before a given batch of emulsionis coated. The exposure. characteristics of the last coated film aresubstantially altered from the characteristics at the start of thecoating. This is especially a problem when fine grain emulsions havingan average grain size of less than about 2 microns in diameter areutilized in the admixture wherein at least one compo nent of the blendhas an average grain size of at least 0.3 micron.

We have now found that silver salt emulsions com prising an admixture ofsilver salt grains of different average diameter sizes can be coatedwith substantially uniform exposure characteristics throughout thecoating run if the individual emulsions to be blended are held inseparate vessels until just prior to coating, e.g., no more than about 2minutes prior to coating. The respective emulsions are vigorously mixedprior to coating with good homogeneity in various mechanical mixingdevices. A typical apparatus suitable for this purpose is disclosed inU.S. Pat. 2,912,343 issued Nov. 10, 1959.

In a preferred embodiment of this invention, directpositive emulsionscomprising various sizes of fogged silver halide grains are coated bythe above process.

In another preferred embodiment, at least two monodispersed silverhalide emulsions, wherein the average grain size of the second emulsionis at least 50% greater than the first, are effectively coated by thisprocess.

In another preferred embodiment of this invention, the respectiveemulsions of different grain size are admixed within one minute prior tocoating.

According to the present invention, the various silver halide portionsto be combined to achieve the improved photographic properties areintimately admixed in the required proportion at or near the cotaingdevice and the mixture is coated within the minimum possible time delaybetween the time of mixing and the time when the emulsion is coated. Theemulsion is then set and dried with the minimum of delay. The emulsionis generally mixed within 2 minutes of coating and preferably within 1minute of the coating of the layer. The emulsion is then set and driedto no more than 15% moisture or liquid carrier within about 30 minutes.A typical apparatus for mixing the emulsions is disclosed in U.S. Pat.2,912,343. Typical coating apparatus are disclosed in U.S. Pat.2,761,791.

The process of this invention generally provides improved photographicemulsions with admixtures of any silver salt composition. Good resultsare obtained wherein at least one of the emulsions to be blended has anaverage grain size at least 50% greater and preferably greater than oneother component emulsion of the admixture. In a preferred embodiment,the largest improvements in photographic properties are obtained whereina first emulsion having an average grain size of at least 0.3 micron isblended with another emulsion having an average grain size which differsby at least 50% in average size from said first emulsion.

Grain sizes can be measured by methods commonly used in the art for thispurpose. A typical procedure is set forth by Loveland, Methods ofParticle-Size Analysis, ASTM Symposium on Light Microscopy 1953, pages94-122, or in chapter 2 of The Theory of the Photographic Process, Meesand Jones, Third Ed. (1966), published by Macmillan Co. The grain sizecan be measured using the projected areas of the grains or approximatediameter. When the grains are substantially uniform in shape, the sizedistribution can be expressed quite accurately as either diameter orprojected area.

The process of this invention is generally effective with any type ofsilver halide composition. However, especially good results have beenobserved when coating direct-positive silver halide emulsions. Typicalemulsions 3 of this type are disclosed in Berriman, U.S. 'Pat. 3,367,778issued Feb. 6, 1968, Illingsworth, U.S. Pat. 3,501,307 issued Mar. 17,1970, Illingsworth, U.S. Pat. 3,501,306 issued Mar. 17, 1970, andIllingsworth and Spencer, Belgian Pat. 695,364 issued Sept. 11, 1967corresponding to U.S. Pat. 3,501,310 issued Mar. 17, 1970.

In a highly preferred embodiment, monodispersed silver halide emulsionsare coated by this process. Typical direct-positive silver halideemulsions of this type are dis closed in Illingsworth, U.S. Pat.3,501,305 issued Mar. 17, 1970. Generally, in such emulsions, no morethan about 5%, by weight, of the silver halide grains smaller than themean grain size, and/or no more than about 5%, by number, of the silverhalide grains larger than the mean grain size, vary in diameter from themean grain diameter by more than about 40%. Preferred direct-positivephotographic emulsions of this invention comprise fogged silver halidegrains, at least 95%, by weight, of said grains having a diameter orprojected area which is within 40%, preferably within about 30%, of themean grain diameter or mean projected area, respectively. Average grainsize can be determined using conventional methods, e.g., as shown in anarticle by Trivelli and Smith entitled Empirical Relation betweenSensitometric and Size-Frequency Characteristics in PhotographicEmulsion Series, in The Photographic Journal, Vol. LXXIX, 1939, pages330-338. The aforementioned uniform size distribution of silver halidegrains is a characteristic of the grains in mono-dispersed photographicsilver halide emulsions. Silver halide grains having a narrow sizedistribution can be obtained by controlling the conditions at which thesilver halide grains are prepared using a double run procedure. In sucha procedure, the silver halide grains are prepared by simultaneouslyrunning an aqueous solution of a water-soluble silver salt, for example,silver nitrate, and a water-soluble halide, for example, an alkali metalhalide such as potassium bromide, into a rapidly agitated aqueoussolution of a silver halide peptizer, preferably gelatin, a gelatinderivative or some other protein peptizer. The pH and the pAg employedin this type of procedure are interrelated. For example, changing onewhile maintaining the other constant at a given temperature can changethe size frequency distribution of the silver halide grains which areformed. However, generally the temperature is about 30 to about 90degrees C., the pH is up to about 9, preferably 4 or less, and the pAgis up to about 9.8.

In some instances, the photographic properties of the elementscontaining the blended emulsion according to this invention can befurther improved by coating a layer containing a water-soluble halideadjacent the blended emulsion layer. Preferably the pAg of the silverhalide layers of the final element is raised to a pAg of above about 9.0by this technique to obtain improved photographic properties.

The process of this invention can also be extended to incorporate colorcouplers into the silver halide emulsion during the mixing step. Theincorporation of couplers or coupler solvents in direct-positiveemulsions of the type in which a silver halide grain has awater-insoluble silver salt center and an outer shell composed of afogged water-insoluble silver salt that develops to silver withoutexposure, prepared in various ways such as those described in Berriman,U.S. Pat. 3,367,778 issued Feb. 6, 1968, improves incubation stability;especially improved is the Dmax. loss observed upon incubation, with noloss in speed; this improvement can be obtained when the emulsions areprocessed in black-and-white Elon-hydroquinone developers and colordevelopers. A useful coupler solvent for incorporating couplers insilver halide emulsions is di-n-butyl phthalate and useful couplers arecyan amidophenol couplers such as those described in U.S. Pat. 2,895,826and the combination of an amidophenol coupler and a phenolic cyancoupler of the type described in U.S. Pat. 2,474,293.

The invention can be further illustrated by the following examples.

EXAMPLE 1 A silver bromoiodide emulsion having an average grain size ofabout 0.4 micron is fogged to maximum density as described inIllingsworth, U.S. Pat. 3,501,307 isued Mar. 17, 1970. Similarly 4fine-grained silver bromoiodide emulsions having an average grain sizeof about 0.2 micron are fogged to different sensitivities so as toobtain the desired exposure latitude when combined with thecoarse-grained emulsion. The above melts are combined at the ratio of 48parts of the fogged coarse-grained emulsion to 52 parts of thefine-grained emulsions. The combined melt after being held for 2 hoursis coated on a polyester support at 300 mg. silver/ft. and 360 mg.gelatin/ftF. Over the emulsion layer is coated a gelatin layer at 82 mg.gelatin/ft.

EXAMPLE 2 A fogged coarse-grained silver bromoiodide emulsion asdescribed in Example 1 is prepared. Similarly 4 fogged fine-grainedsilver bromoiodide emulsions as described in Example 1 are combined andthe two melts are mixed, as described in Collins et al., U.S. Pat.2,912,343, at the same ratio as in Example 1 just prior to coating on apolyester support at 300 mg. silver/ft. and 360 mg. gelatin/ft. Over theemulsion layer is coated a gelatin layer at 82 mg. gelatin/ft.

The coated samples are exposed for 10 seconds on an Eastman 1Bsensitometer to light from a tungsten source modulated by a continuouswedge and developed for 6 minutes in an Elon-hydroquinone developer.After the samples are fixed, washed and dried the following results areobtained.

It can be readily seen from the above table that the procedure of mixingthe coarse-grained and fine-grained emulsions just prior to coatingresults in a higher contrast, higher D and a lower D for a givenexposure range.

Although the invention has been described in considerable detail withparticular reference to certain preferred embodiments thereof,variations and modifications can be effected within the spirit and scopeof the invention as described hereinbefore and in the appended claims.

We claim:

1. In a method of making a photographic element having at least onelayer containing a blend of fogged, directpositive silver halideemulsion wherein the average grain size of at least two component foggedemulsions of the blend difiers by at least 50%, which method comprisespreparing the respective component fogged, silver halide emulsions,blending said component silver halide emulsions, coating said emulsionblend on a support and reducing the liquid content of the coated blend;the improvement which comprises (1) deferring the blending until justprior to coating, (2) coating said blend on a support within about 2minutes after blending and (3) drying to less than 15% carrier liquidcontent within 30 minutes.

2. A method according to claim 1 wherein at least two component foggedemulsions of said blend differ by at least 100% in average size.

3. A process according to claim 1 wherein said component silver halideemulsions are each monodispersed, fogged, silver halide emulsions.

4. A process according to claim 3 wherein at least one of said componentemnulsions has an aver-age grain size of less than 2 microns.

5. A process according to claim 1 wherein said blend is coated on asupport within at least 1 minute after mixing said component emulsions.

6. A process according to claim 1 wherein at least one of said componentemulsions has an average grain size of at least 0.3 micron and at leastone other component of the blend has an average size which differs by atleast 50% in average size.

6 References Cited UNITED STATES PATENTS OTHER REFERENCES Baker,Photographic Emulsion Technique, pp. 99-100.

GEORGE F. LESMES, Primary Examiner R. E. MARTIN, Assistant Examiner US.Cl. X.R. 9698, 103

